1. Field of the Invention
The present invention relates to a chopper control system, and more particularly to an improvement in a chopper control system which enables reduction of wheel slip.
2. Description of the Prior Art
The rotational speed of a shunt motor changes in proportion to variations in the voltage across the armature when the strength of the field flux is constant.
When a vehicle driven by a shunt motor is normally accelerated or decelerated, it runs with a gradual change in the speed, and therefore, the optimal form of control is an integral one in which the difference between the motor current which flows through the armature and a command current value is integrated, and the supply voltage to the armature is varied in accordance with the thus integrated value.
One of the variable factors in the control of the operation of a vehicle's motor is the fluctuation in the voltage from the power source, for example, from the stringing or line, and this has to be given due consideration. Therefore, in order to obtain a desired tractive force immediately in response to a fluctuation in the line voltage, a conventional chopper control device for a vehicle has a circuit arrangement which exhibits a powerful characteristic of constant current. With this arrangement, however, since the motor current is proportional to the difference (Vm-Em) between the voltage (the mean value) Vm supplied from the chopper control device and the electromotive force Em generated by the armature, whenever any wheel slip takes place between the wheels driven by the motor and the rails on which the vehicle is running, the electromotive force Em will change and, the motor current will also vary in accordance with this change. In general, whenever wheel slip takes place during traction or braking of the vehicle, the motor current has to be reduced temporarily in order to compensate for the wheel slip which has taken place. However, the conventional chopper control device which has a constant current characteristic operates to control the motor current in such a manner as to consistently maintain the same at a constant level, thus failing to reduce the degree of wheel slip, and as a result, acceleration or deceleration control is performed with difficulty in the vicinity of the force limit of adhesion between the wheels and the rails.